A known type of battery is the so-called ZEBRA battery. Additional information regarding the “ZEBRA” battery (and a similar sodium/sulfur battery) can be found in the published literature, including Karina B. Hueso et al., “High temperature sodium batteries: status, challenges and future trends,” Energy Environ. Sci., 14 Jan. 2013 and J. L. Sudworth, “The Sodium/nickel chloride (ZEBRA) battery,” J. Power Sources, 100 (2001) pp. 149-163.
FIG. 1 shows a drawing of an exemplary cathode and anode that is used in a “ZEBRA” battery 100. The battery 100 includes a Ni cathode 116 and a molten Na metal anode 114. Electrolyte is used to carry charge between the anode and the cathode. This electrolyte may be a sodium haloaluminate (NaAlX4) material such as NaAlCl4, NaAlBr4, or NaAlI4. A NaSICON or Beta alumina membrane 115 is used to separate the anode from the electrolyte. The charge-discharge reactions are summarized below, for the embodiment where the halogen is chlorine:(Anode) 2Na2Na++2e−(Cathode) NiCl2+2Na++2e−Ni+2NaCl(Overall cell) NiCl2+2NaNi+2NaCl, E=2.58V
The foregoing oxidation/reduction reactions of Na and Ni produce the charge within the battery 100. The cathode is fabricated of a porous structure of nickel. The pores are impregnated with an electrolyte comprising NaAlCl4. NaAlCl4 has a melting point of about 157° C. Thus, the ZEBRA battery must be operated at a sufficiently high temperature to ensure that the NaAlCl4 (or other sodium haloaluminate) is molten with a sufficiently low viscosity to enable penetration of the porous nickel cathode and sufficiently high conductivity of sodium ions.
In this battery during discharge, Na is oxidized at the anode to form sodium ions that transport across membrane 115. Nickel halide is converted to metallic nickel and sodium halide which becomes part of the molten NaAlX4 electrolyte. During charge, sodium ions are transported across membrane 115 and reduced at the anode. The Ni cathode is oxidized to NiX2.
One of the features of the ZEBRA battery is that it is typically operated at 300° C. or higher due to the ohmic resistance of the membrane 115 and the need to have the NaAlX4 be molten. The high temperatures are used to ensure that the NaAlX4 is molten and has a low viscosity so that the Na+ ions may transport between the cathode materials and the membrane 115.
At about 300° C., the bond between the Na ion and the [AlX4]− moiety weakens, as shown below:Na+ - - - [AlX4]−The dashed line indicates that the bond weakens such that, for some of the NaAlX4 species, the bond will actually break, thereby allowing Na+ ions to transport. It is this weakening/breaking of the bonds that allows the battery to operate.
However, operating the ZEBRA battery at 300° C. can be cost-prohibitive as it requires a tremendous amount of energy to maintain the battery at that high temperature. Accordingly, there is a need in the art for a new type of battery that is similar to the ZEBRA battery that may be operated at lower temperatures. Such a device is disclosed herein.